1. Field of the Invention
The present invention relates to a metal golf shaft a to a golf club.
2. Description of Related Art
The weight of a metal golf shaft is desirably reduced in order to increase the flight distance of a golf ball. This desire has been met by reduction in wall thickness of a golf shaft having a hollow tubular shape. However, since it is necessary to satisfy the SG standard for ensuring the safety golf shafts, strengthening of the material in golf shafts is necessary in order to reduce the wall thickness thereof.
For example, Japanese Unexamined Patent Application Publications Nos. 2005-34517 and 2005-13535 disclose techniques for strengthening a material of a metal golf shaft. In the technique disclosed in Japanese Unexamined Patent Application Publication No. 2005-34517, an alloy type steel product having a high strength and a high toughness is subjected to sub-zero treating, and a material of a metal golf shaft thereby has a martensite structure. Next, the material of the metal golf shaft is subjected to tempering, so that the toughness thereof is restored. As a result, the metal golf shaft can have strength and toughness. In the technique disclosed in Japanese Unexamined Patent Application Publication No. 2005-13535, amount of retained austenite included in an alloy type steel product having a high strength and a high toughness is adjusted, and a metal golf shaft having a light weight can be superior in strength, toughness, elasticity, and rebound force, and the like.
In the conventional techniques, in order that a material of golf shaft is uniform in a thickness direction and in a longitudinal direction, material composition, metal structure, grain size, and hardness are adjusted so that the material of golf shaft is superior in balance of strength and toughness. In the conventional techniques, the material of the golf shaft had to be substantially uniform in a thickness direction and a longitudinal direction regardless of the magnitudes of stresses generated to the golf shaft. However, when a golf shaft is used under a bending stress, the stress is maximal at a surface of the golf shaft. In addition, a high stress is applied to a grip side of the golf shaft. Thus, when a material is adjusted so as to optimize a portion to which a high stress is applied, a portion to which a low stress is applied has an excessive quality. Since retained austenite has a soft structure, the strength and the proof stress are decreased, inferior effects may be obtained. When the amount of retained austenite is increased, the toughness is improved, but the strength is decreased. Due to this, it is difficult to simultaneously improve both the strength and the toughness.